There are a few different things that people might mean when they say their phone is "locked". I wanted to provide this post to help define the different types of lock.
This typically means that the phone will only work on the network of one specific operator. This is handled by the GSM network and doesn't have anything in particular to do with the Windows Mobile software itself. More info
In many cases, Windows Mobile devices can only be reflashed by the OEM or operator. The "bootloader lock" typically refers to this state. The bootloader is the very first part of the phone that starts up, and it usually has the capability to enter a reflash mode or various other debug or diagnostic modes.
This term is a little bit ambiguous. In the most strict meaning, it means that the device can't run unsigned code and that most settings and the registry entries can't be changed by the end user. Sometimes the term is used to refer to the default settings on a two-tier Smartphone device, where unsigned code runs at the "normal" trust level and only some settings and APIs can't be used. If there is interest, I can do a followup post with more detail on what this means and ways to tweak this state on development devices.
PIN Locked (or Device Locked)
This refers to the device state where you need to enter your PIN (or password) to unlock and use the device. Internally we call it "device lock" but "PIN lock" is probably more clear. There's also "key locked" where most keypresses don't register except the unlock key.
SIM PIN Locked
The SIM itself can require a PIN in order to change settings or use the phone features at all. If the PIN is entered incorrectly enough times, the SIM will lock down and you have to get an unlock code (PUK) from the operator to continue. This functionality is implemented entirely on the SIM - all the Windows Mobile software does is pass the PIN code from you to the SIM
I'm moderating this post, so please don't post links to various unlock sites and resources - I won't post those comments for various different reasons including our partner relationship with OEMs and operators.